via the Montague Tunnel, no... there's no reason to unless after Phase 4, the job market in the general area of Seaport increases drastically, but as of now, Midtown is the main hub, so it would be more ideal to have a Manhattan Bridge connection. That being said, however, this would also mean that either Broadway/6 Av service on the bridge would have to be replaced, and there is no easy way to do that without extensive construction. I would just not let the (or whatever) get caught up on 4 Av... building a connection to the Fulton Lcl tracks would be beneficial to riders however and it would help clear up that merge on Fulton.
Hopefully, there will also be a seamless cross-platform transfer at Grand St.